This invention relates to a complexing agent and in particular to new fluorinated thiourea complexing agents and fluorinated thiourea complexing agents for use in the extraction of noble metals such as gold, platinum, silver, palladium and rhodium.
Complexing agents are widely employed in the extraction and recovery of metals such as gold, platinum, silver, palladium and rhodium.
For example, gold is a soft yellow metal having a melting point of 1063° C. with the highest ductility and malleability of any element. It is chemically unreactive and is not attacked by oxygen or sulphur but reacts readily with halogens or with solutions containing or generating chlorine such as “aqua regia”. Its most common compounds exist in the (I) and (III) oxidation states.
Heretofore, the extraction of gold from one and from other solid phases such as in solid phase extraction has been commonly carried out by using cyanide or thiourea as reagents. In the most commercially important method for gold extraction finely crushed ore is treated with sodium cyanide in the presence of oxygen to give a sodium gold cyanide complex, which is typically absorbed onto activated carbon. The sodium gold cyanide complex can be re-extracted later and reduced to the metal, (H. Schmidbaur, Interdisciplinary Science Reviews, 17 (3), 213, 1992 and A. Sigel and H. Sigel in “Handbook on Metals in Clinical and Analytical Chemistry”, Ed. H. G. Seller, 1994 p388) viz:4Au+8CN−+O2+2H2O→^4[Au(CN)2]−+4OH31 
However, treatment with sodium cyanide is environmentally unfriendly while the efficiency of the reaction can be poor and variable according to the ore type. Accordingly, other methods of gold and silver extraction have been developed e.g. thiourea-based extraction. Thiourea-based extractions enjoy the advantages of higher leaching efficiency, rapid leaching, adaptation to a variety of refractory ores and reduced toxicity to the environment. Accordingly, thioureation is an attractive procedure for the extraction of both gold and silver.
For example, it has been demonstrated (C. K. Chen, T. N. Lung and C. C. Lung and C. C. Wan, Hydrometallurgy, 5, 207, 1980) that employing Fe3+ as oxidant in acid solutions resulted in leaching with thiourea which was ten times faster than leaching with sodium cyanide, viz: 
However, excessive consumption of thiourea in the process has limited its industrial application.
Various attempts have been made to reduce thiourea consumption. For example, in order to reduce thiourea consumption in gold extraction it has been suggested (C. C. Kenna, Gold Bull, 24(4), 126, 1991) that the complexing of ferric ions could be utilised in reducing their oxidative power to a level where oxidation of gold still proceeded at an acceptable rate while oxidation (and consumption) of thiourea was greatly reduced.
U.S. Pat. No. 5,126,038 also discloses that alkyl hydroxamic acids or their salts may be used to improve extraction of precious metals, including gold, from sulphide ores in combination with standard sulphide ore collectors such as xanthates, substituted thioureas and the like.
G. Zuo and M. Muhammed, Separation Science and Technology, 25(13-15), 1785, 1990 also describe the synthesis and characterisation of a family of thiourea based reagents for the extraction of Au(III) and Ag(I) ions through complex formation from HCl solutions and also disclose the synthesis of several co-ordinating polymers by grafting thiourea functional groups onto commercial macroporous polystyrene polymer matrices.
In order to avoid the use of thioureas, azacrowns have also been used to facilitate transport of NaAu(CN)2 into an organic phase from an aqueous phase (M. Tromp, M. Burgard, M. J. F. Leroy and M. Prevost, J. of Membrane Science, 38, 295, 1988). In addition, Izatt et al., (R. L. Bruening, B. J. Tarbet, T. E. Krakowiak, M. L. Bruening, R. M. Izaat and J. S. Bradshaw, Anal. Chem., 83(10), 1014, 1991 and R. L. Bruening, B. J. Tarbet, K. E. Krakowiak, R. M. Izatt and J. S. Bradshaw, J. Heterocyclic Chem., 27 347, 1990) have developed silica gel bound thia—macrocycles which have shown high selectivity for Au(III).
Supercritical fluid extraction (SFE) has developed into an attractive alternative to conventional solvent extraction to recover organic compounds from solids in particular. A useful fluid for SFE work is liquid carbon dioxide due to its moderate critical constants (Tc=31.1° C., Pc=72.8 atm), inertness, ease of availability, low cost and ease of final removal. However, direct extraction of metal ions by supercritical CO2 is very inefficient due to the change neutralisation required and weak solute-solvent interactions.
Supercritical fluid extraction of gold has been described by S. Wang, S. Eishoni and C. M. Wal, Anal. Chem., 67, 919 1995 where Au(III) ions were extracted by bis-triazalocrowns from wet solid matrices using supercritical CO2 modified with methanol. Neutral gold complexes were formed due to the presence of triazalo protons: which were soluble in modified SF—CO2. The presence of the triazolo protons was necessary for the extraction of the metal ions to give a neutral metal ion-ligand complex: and no extraction was possible without methanol modifier or water in the solid phase. Supercritical CO2 has also been utilised (E. O. Out, Separation Science and Technology 32, 6, 1107, 1997) to elute gold in the form of NaAu(CN)2 previously adsorbed on activated charcoal employing tributylphosphate to facilitate charge neutralisation. However, the presence of water in the solid phase was required for the extraction while are indicated previously the use of cyanide is undesirable for environmental and safety reasons.